Liquid-cooled, segmented glass laser

ABSTRACT

Laser device of the neodymium doped glass disc array type. The discs are spaced apart from one another and are immersed in a liquid whose refractive index is equal, for the emission wavelength of the laser, to the refractive index of the glass forming the discs. The disc inclination angle with respect to the disc alignment direction can take any desired value. To pump the discs in the whole volume thereof, the spacing is given a value higher than a predetermined minimal value and an inclination angle of 45* is selected. The immersion liquid is a mixture of a brominated saturated acyclic hydrocarbon and a saturated acyclic alcohol.

United tates Patent Gans May 22, 3973 LIQUID-COOLED, SEGMENTED GLASSPrimary Examiner-William L. Sikes LASER Attorney-Abraham A. Saffitz [76]Inventor: Francois F. Gans, 38 rue Gustav Vatonne, Gif-sur-Yvette,France [57] ABSTRACT Laser device of the neodymium doped glass discarray [22] June 1972 type. The discs are spaced apart from one anotherand {21] App]. No.: 262,600 are immersed in a liquid whose refractiveindex is equal, for the emission wavelength of the laser, to the 52 us.CL... ..331/94.5, 330/43 f l of the .glass formmg the i 5! I 0 H01 CllSCinclination angle with respect to the (118C alignn 5 3 ment directioncan take any desired value. To pump 1 0 care l the discs in the wholevolume thereof, the spacing is given a value higher than a predeterminedminimal [56] References C'ted value and an inclination angle of 45 isselected. The

UNITED STATES PATENTS immersion liquid is a mixture of a brominatedsaturated acyclic hydrocarbon and a saturated acyclic al- 3,487,33012/1969 Gudrnundsen ..331/94.5 cohol 3,675,152 7/1972 Young ..33l/94.5

3 Claims, 5 Drawing Figures x I I 12 i L 7 2 j P i H H H Pmmd m 22,1:733,735,282

2 Shah-Shut 1 4 Fig.1 J

Fig 2 LlQUlD-COOLED, SEGMENTED GLASS LASER This invention relates tosegmented solid state lasers and more particularly to high energysegmented neodymium glass lasers using an array of discs.

Laser device using disc-shaped bodies of laser material which arepositioned at the Brewster angle have been recently developed. They havewith respect to conventional lasers using long laser material rodssigniflcant advantages. It is known that high outputs of laser energyare obtained by increasing the pumping energy and the length anddiameter of the laser rod. A diameter limit and a length limit arereached beyond which an increase in the diameter and length of the roddoes not produce a proportional increase in the laser output energy. Asthe depth to which the pumping energy can penetrate is limited, beyond aparticular diameter size the laser energy increases proportionally tothe diameter of the rod and not to the volume thereof and beyond ahigher diameter size the beam takes a somewhat hollow configuration dueto the reduced excitation or the absence of excitation at the center ofthe rod. lncreasing the length of the laser rod presents (i) thepractical problems of producing long pieces of optically perfect lasermaterial, (ii) mechanical and thermal problems, (iii) risks ofdestruction of the laser material at high levels of laser energy densityand (iv) a spontaneous avalanche condition due to high gainlength factorof the laser rod.

A thin laser disc having large end surface areas and inclined withrespect to the laser axis at the Brewster angle (the Brewster angle isin the range of approximately 57 to 60 for conventional solid lasermaterials) can be optically pumped in its total volume since the pumpingenergy can penetrate at the same time the plane faces and thecylindrical face of the disc (pumping energy can penetrate in theportion of the plane faces of each disc not masked by the adjacent discwith respect to the flash lamp). Thus the laser beam generated by thelaser disc does not have the undesired hollow configuration which therod-type laser may have due to unexcited central portions. Further, thedisc having a larger diameter than the rod, high energy operation cantake place at much lower energy density, thereby avoiding the danger ofdestruction of the laser material. Finally, the gain-length factor ofthe laser disc being small prevents the spontaneous avalanche conditron.

It has also been proposed to cool a segmented laser structure by acoolant fluid and at the same time to avoid any losses at the interfacebetween the coolant and the laser material by matching their refractiveindices at the laser wavelength and to select as coolant fluid a coolanttransparent at both the laser and pumping wavelength and stable underthe pumping illumination. in function of the laser material, the coolantcan be selected from the group consisting of water, heavy water, methylalcohol, benzine and freon, a solution of CaCl in heavy water and asolution of baryum iodomercurate in heavy water.

Neodymium-doped crown glass has a refractive index of about 1.512 to1.520 at C for the wavelength 1.058 pm. The coolant fluid must have sucha high refractive index and additionally must be transparent at theemission wavelength of 1.058 um and at the pumping wavelength and mustnot be submitted to photolyse, i.e. to decomposition, by photonicillumination in the spectral region of the pumping lamp radiation. Thepumping lamps are generally either high pressure mercury arc light lampsor xenon or krypton flash lamps all having significant radiations in theultra-violet spectral region.

Attempts to find liquids or passive liquid mixtures meeting theabove-mentioned conditions have been unsuccessful. The refractive indexof water (1.324) at the laser wavelength and heavy water (1.323) aredefinitely too small and these material absorb the light at 1.06 m, thesecond with a degree 10 tiines lower than the first. Saturated alcoholsused alone have refractive indices by far smaller than that of neodymiumdoped glass for instance, 1.31 for methanol and 1.38 for ethanol insteadof 1.52 for neodynium doped glass. Barium iodomercurate and bariumbromomercurate, respectively Ba[l-lgl and Ba[HgBr are decomposed after atime by ultra-violet radiations with production of free mercury andtheir refractive index, if it can match that of neodymium-doped calciumfluoride (1.428 at 1.06 pm) is by far smaller to the desired index of1.52. Benzene (refractive index 1.47) and more generally liquidunsaturated organic substances are photolyzed by ultra-violetradiations.

I have found that bromo-substituted saturated acyclic hydrocarbons reactwith saturated acyclic alcohols when illuminated by ultra-violetradiation and that the reaction is reversed during the intervals betweenillumination pulses. Although the actual mechanism of the reaction isnot certain and 1 do not wish to be restricted to the explanationthereof, I believe that the following reactions take place, taking asexample of bromosubstituted saturated hydrocarbon, bromoform CHBr and assaturated acyclic alcohol, anhydrous ethanol C H OH. Ultra-violetphotons dissociate CHBr into (i) Cl-[Br and Br (atomic) and (ii) CBr andHBr CHBr hv(U.V.) CBr HBr Atomic bromine Br converts into molar bromineBr Br Br Br in acid medium due to HBr, Br reacts with C H OH Br C l-[ 0HC H Br BrOH Reactions (1) and (2) are reversed in the absence ofultraviolet radiation and reaction (5) is reversed when the temperaturefalls under about C. Accordingly, bromoform and ethanol which react oneach other during illumination, reform during the pulse intervals,except for a small quantity of hypobromous acid which forms quite slowlyand progressively decreases the transparency of the index matchingliquid to the pumping radiation between 5000 and 8800 A.

The bromo-compounds must have at least three bromine atoms per moleculein order for the refractive index to exceed the predetermined value of1.52. The amount of alcohol is such that the mixture has a refractiveindex of 1.52. In the case of bromoform and ethanol, the weightcomposition of the liquid mixture is 60 percent bromoform, 40 percentethanol. Further the alcohol must have at least two radicals (OH) for 10atoms of bromine in order to give to the mixture a life expectancy of100 hours.

It is therefore the object of the present invention to provide a laserstructure utilizing segmented neodymium-doped glass and cooled by aliquid having a refractive index identical to that of said glass, saidliquid remaining transparent to the pumping illumination despite theU.V. radiation comprised therein.

The invention will now be described in detail with reference to theaccompanying drawings in which FIG. l is a partial, sectional elevationof a laser structure having a segmented laser rod immersed in a coolantand index matching liquid FIGS. 2, 3a and 3b illustrate details of theapparatus of FIG. I and FIG. 4 represents a laser of the invention whichparallel discs at 45 with respect to the laser axis.

Referring to FIG. 1, the laser proper is placed inside athermostatically controlled housing 1 and includes a tube or tank 2containing the active segmented laser medium, a helical optical pumpingarc lamp 3 ending with electrodes 4 and containing krypton or xenon or amelange thereof, and caps 5 to close the tube 2. The tube 2 and thehelical arc lamp 3 are mounted coaxially. A hollow member 6 surroundsthe arc lamp 3 and the tank 2; it may be mirrored on its surface to aidreflection of light from the arc lamp into the lasering material in thetank.

The tube 2 is a "Pyrex" glass tube of 3 cm. internal diameter and 1.5mm. thickness. It may be coated externally with an ultraviolet-absorbinglayer and is equipped with a side-arm 7 to eliminate gas bubbles fromthe liquid it contains.

The tube 2 is filled with a number of neodymiumdoped crown glass discs8, 3 cm. in diameter and 2 mm. thick. The refractive index of this glassfor the wavelength l.058 m is 1.512 at 20C. The discs are placed in aliquid mixture 9 of 60 percent bromoform and 40 percent ethanol. Thewall of the tube 2 containing the discs 8 and the liquid 9 is coatedwith an ultra-violetabsorbent layer, but this coating is not necessary.

The spacing between the discs if not critical. They may be set in a comb10 as shown in FIG. 3a. They are apertured at their periphery to allowthe cooling fluid to circulate (see FIG. 3b).

The laser device is placed on an optical bench ll between two mirrors12. Each end of the tube 2 is closed by a neodymium-doped crown glasswindow 13. This window is clamped between two threaded joints M and withinterposition of flat joints H6 in polytetrafluorethylene, the threadedjoints ending in tubular sections 17 and 18. Onto the tubular section 18can be screwed a threaded ring 19 in which the tube passes and whichserves to grip an O-ring seal 20.

An ultra-violet-absorbing substance is mixed with the liquid in thethermostat l to enhance the effect of the layer covering the tube 2 andprevent photolysis of the liquid it contains. This absorbant can be amixture of cerium and ammonium nitrate dissolved in water in the ratiol/l00 by weight, which eliminates radiations of wavelength below 0.46pm.

Referring now to FIG. 4 the tube 22 contains neodymium glass ellipticaldiscs 28 which are oriented at 45 with respect to the axis of the tube.The discs 28 are held in position by a comb 30 having inclined goovesand which can be separated into two halves for mounting purposes. Thetube 22 is provided with an inlet 31 and an outlet 32 for allowing anappropriate refractive index liquid to circulate by means of a pump notshown and simultaneously cool the emissive discs.

The active tube is illuminated by two flashlamps 23 and 24 parallel tothe axis of tube 22 and located in the plane passing through theprojection of the tube axis onto the planes of the discs. The flashlampsand the tube containing the segmented neodymium glass are surrounded bya mirror 25. Ifd and e respectively designate the diameter and thicknessof the discs, the spacing therebetween is taken at least equal to (dV274 e 2). It results that each disc is thus pumped by the lamps up toits center, the first flashlamp pumping a half volume of the disc andthe other flashlamp the other half volume. Under these conditions, theweight percent doping in Nd O can be increased with respect to the priorart, say from 3 to 5 percent.

Bromoform has a refractive index of 1.598 at 1.06 um and ethanol anindex of l.38.

A second example of coolant and refractive index matching liquid isgiven hereinafter. It comprises 64 percent 1,2-tribromoethane which hasa refractive index of 1.588 and 36 percent of propanol having arefractive index of L39.

The laser of the invention has been tested as regards its expectedlifetime. The coolant fluid continuously irradiated by a 250 watt highpressure xenon arc lamp has experienced a decrease of transparency of 5percent in the 5000-8800 A range of pumping illumination after a test ofI00 hours.

Since the bromined compounds on the one hand and the alcohols on theother hand have very close refractive index, the mixture generallycomprises from 55 to 65 percent of the first compound and from 45 to 35percent of the second component.

I claim:

I. In a laser structure comprising an elongated tube, neodymium dopedglass discs inserted in said tube and spaced apart therebetween, meansfor holding said discs parallel to one another, a coolant liquidsubstance filling in said tube and in which the discs are immersed, saidliquid substance having a refractive index matching that of the glassforming the discs for the wavelength radiated by the laser device andmeans for optithe percentage of about 55 to 65 percent and the saturatedacyclic alcohol is ethanol in the percentage of about 45 to 35 percent.

3. In the laser structure of claim 1, the bromine sub stituted acyclichydrocarbon is 1,2 tribromoethane in the percentage of about 55 to 65percent and the saturated acyclic alcohol is propanol l in thepercentage of about 45 to 35 percent.

2. In the laser structure of claim 1, the bromine substituted acyclichydrocarbon is bromoform CHBr3 in the percentage of about 55 to 65percent and the saturated acyclic alcohol is ethanol in the percentageof about 45 to 35 percent.
 3. In the laser structure of claim 1, thebromine substituted acyclic hydrocarbon is 1,2 - tribromoethane in thepercentage of about 55 to 65 percent and the saturated acyclic alcoholis propanol 1 in the percentage of about 45 to 35 percent.